In classical music I use envelope to make soft parts loud and loud parts soft,
but envelop is not powerful enough.
I can increase volume of a section, but then the piece doesn’t sound natural.
So I want to use envelope, save project and forget previous envelope and
restart enveloping. Is this possible?
There is an ugly (time consuming) way: Modify, export, and edit the output again.
Also I’m worried quality of the sound may be lost with too much editing.
If I change the speed more than once, does it hurt quality of the piece? (probably not)
What if I change the pitch orthe duration more than once? (probably /unfortunately? yes)
You can use envelope, then ‘mix and render’ track and envelope the result again.
You can do this several times and it doesn’t hurt the sound quality.
Changing speed can cost quality because of the resolution being limited (usually 44100 Hz sample rate)
(Same happens with resizing pictures)
An alternative approach would be to use a “dynamic compressor” effect.
Audacity includes a compressor Compressor - Audacity Manual
There are also 3rd party plug-ins available. A popular one is: https://forum.audacityteam.org/download/file.php?id=24860
This is a “Nyquist plug-in”. Installation instructions are here: https://manual.audacityteam.org/man/installing_effect_generator_and_analyzer_plug_ins_on_linux.html#nyquist_install
Please note that the “Nyquist Plug-in Installer” is only available in Audacity 2.3.1 and later. For earlier versions of Audacity, you have to put the plug-in in the correct place yourself. “Per user installation” is generally recommended.